Expansion of Chenopodium Album Leaf Disks as Affected by Coumarin.
Although leaf expansion is obviously a fundamental process in the development of plants, its physiological mechanism is little understood, and has been but sparingly investigated. It is known that auxins affect the elongation of the leaf veins (1) but do not seem to influence the expansion of the laminar leaf tissues. BONNER et al. (3) observed an increase in expansion of radish leaf disks floated on solutions of adenine. BONNER (2) also found potassium nitrate to be effective in promoting expansion and (private communication) believes the salt to have both a potassium and a nitrate effect. de Ropp found several substances, including adenine, to be without effect on elongation of first leaves of isolated rye stem tips (4), and several substances to be without effect on the final dry and wet weights of cabbage leaf disks (5). Various extracts and solutions injected into the hollow petioles of squash plants did not increase the expansion of the leaves in the dark (7). In an effort to achieve a better understanding of the physiology of leaf expansion, a number of growth inhibitors have been tested. Coumarin is one of these compounds. Coumarin affects Avena root elongation (6), elongation of Avena coleoptile sections, and curvature of split pea stem internodes (8). In the papers cited, emphasis has been placed on growth inhibition by the compound. However, in the latter work, increases in elongation of Avena coleoptile sections were obtained at concentrations lower than 10-4 M (10-4 M is approximately equal to 15 p.p.m.). In the work reported here, coumarin in concentrations ranging from 1 to 200 p.p.m. has been found to increase expansion of leaf disks taken from Chenopodium album plants growing outdoors during July, August, and September, 1950. Disks, 5.0 mm. in diameter, were cut with a cork borer from immature leaves having areas of approximately 250 sq. mm. The largest leaves on the plants were about eight times this size. Two disks were cut from each leaf, one from the basal section on each side of the midvein. A segment of a main lateral vein constituted a diameter in each disk. Sixteen disks with lower epidermis up were floated on 10 ml. of the various solutions in Petri dishes. The dishes were placed in the dark and kept at 25 ± 10 C. The check medium contained 4%o D-glucose by weight and KNO3 at 0.2 M. In preliminary experiments, these concentrations were found to be optimal. The pH of each solution was 5.6. At the end of the test period, the disk diameters, perpendicular to the segments of lateral veins, were remeasured.
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